HLG was developed to be compatible with standard SDR tvs. It's getting patched into the B6 model as well, in Europe only for now as the BBC iPlayer is the only thing that uses it so far. Sony has been updating their firmware as well to support HLG. HLG is simply a different (standard) curve for HDR to put more detail into the SDR range of the content. Basically a trade off, less definition in the highs for better definition in the SDR range.
HLG, or Hybrid-Log Gamma, is a one of the newer standards on the market, but it's an entirely different beast from Dolby Vision and HDR. HLG was developed by the BBC and NHK broadcasting networks to serve as an HDR format for live video. Unlike other HDR methods, which pre-encode the content with metadata to properly display the HDR effect, the HLG system is designed to work similar to regular broadcast television. It simply includes additional information regarding the HDR effect that compatible sets can implement. The broadcast is also backwards compatible with older standard dynamic range images should the set not offer HLG compatibility.
While HLG is still years away from any mainstream rollout, there’s nothing about the spec that would prevent any HDR set from offering a firmware update to support it later on.
Thanks for the insight on HLG. For some reason I thought it was the new format for HDR 10 that suppose to have the dynamic metadata that is lacking in the format that Dolby Vision has which allows it to adjust the brightness level per frame. I believe the dynamic metadata is the big win for Dolby Vision at the moment as it does give the extra pop when transitioning from day to night scenes.